There are fundamental theoretical limitations on any conventional ICA-based (independent component analysis-based) or AMUSE-Based blind separation methods. These are due to their requirement that the number of observations (e.g., electrode pairs) must be less or equal to the number of uncorrelated signal sources.
In the fECG (fetal electrocardiogram) extraction, this is not the case. Each electrode has its own, at least partly uncorrelated noise, due to maternal abdominal myoelectric activity and other noises picked up by these electrodes and which differ per each abdominal electrode placement. Also, the mECG (maternal electrocardiogram) and fECG in each channel are partly uncorrelated. Hence, adding electrodes will increase the number of noise sources which are not negligible (amplitude-wise) in weeks 32 of gestation and earlier, thus preventing adequate noise filtering via ICA-based or AMUSE-based blind separation methods. Furthermore, the mECG is of many times (even hundreds) of times stronger that the fECG signal embedded in it and especially than the P or T components of the fECG. Finally, the various noises in which the fECG is embedded may also be stronger by a factor of ten or more (depending on gestation age) and more so with respect to its P or T components.
Whereas Doppler ultrasound methods allow detection of fetal heartbeat down to the 10th-12th gestation week, no ultrasound method can yield useful time recordings of fECG. However, access to reliable and easily extractable fECG recordings would help the physician to better determine fetal heart condition and in some cases, diagnose fetal cardiac defects early in the pregnancy. Hence, in some cases, these can be treated by medication administered to the mother.